1. Field of the Invention
The present invention relates generally to adjustable resistors and, more particularly, to polysilicon resistors that can be electrically adjusted to a precise resistance value.
2. Description of Related Art
Resistors with precise resistance values are useful for a variety of applications. FIG. 1 shows a typical prior art differential amplifier, which is one application where precision resistors can be used. Differential amplifiers have been known in the prior art and are used to multiply the difference between two inputs of the amplifier by a constant factor. The differential amplifier shown in FIG. 1 includes an operational amplifier (i.e., “op amp”) 10, resistors 12, 14, 16, and 18, and voltage source VIN. The inverting input of the op amp 10 is connected to the junction of the pair of resistors 12 and 14, which are disposed in series between the negative output of VIN and the output of the op amp 10 (shown as VOUT). The non-inverting input of op amp 10 is connected to the junction of the pair of resistors 16 and 18, which are disposed in series between the positive output of VIN and ground (GND). Resistors 16 and 18 are also used to remove amplifier offset. Ideally, the ratios of resistor 14 to resistor 12 and resistor 18 to resistor 16 should be equal. When the ratios are equal, the output voltage VOUT will not change when the inverting and non-inverting inputs are tied together and a voltage VIN is applied to both inputs.
In practice, however, it is difficult to manufacture a polysilicon resistor with a precise resistance value. Polysilicon resistors are simple and inexpensive to fabricate, but their resistance values can change with applied voltage and temperature. Polysilicon resistors generally have resistance tolerances ranging from 15 to 20%. When the resistor ratios in the differential amplifier discussed above are not equal to each other, a common mode error (CME) will result. The magnitude of the CME is a measure of the inability of a differential amplifier to block common-mode components of a signal while amplifying the differential signal. CME is an important parameter in applications where the signal of interest is superimposed on a voltage offset or when relevant information is contained in the voltage difference between two signals.
Precise resistance values are important in other applications as well, including precision measurement devices, such as the ones described in the commonly-owned patents, U.S. Pat. No. 6,828,775, issued Dec. 7, 2004, entitled “HIGH-IMPEDANCE MODE FOR PRECISION MEASUREMENT UNIT,” and U.S. Pat. No. 7,154,260, issued Dec. 26, 2006, entitled “PRECISION MEASUREMENT UNIT HAVING VOLTAGE AND/OR CURRENT CLAMP POWER DOWN UPON SETTING REVERSAL,” which are incorporated herein, in their entireties, by reference. The precision measurement units described in these patents generally relate to the field of automatic test equipment for semiconductor devices. Precision resistors are helpful in obtaining the precision measurements required in the automatic test equipment.
Various methods have been used in the prior art to achieve precise resistance values. One method is to use an adjustable component such as a potentiometer, which is a type of variable resistor. A designer would use a potentiometer during testing until the desired function of the circuit had been reached. When used in a differential amplifier as shown in FIG. 1, the potentiometer can be adjusted so that the common-mode signal is nearly completely rejected. One disadvantage of using a potentiometer is cost, particularly when very expensive potentiometers must be used for high-precision differential amplifiers. Another disadvantage is that long-term stability is difficult to achieve with the use of potentiometers.
Another method of obtaining a precise resistance value for thin-film metal resistors is through laser trimming. Laser trimming is the controlled alteration of a capacitor or resistor geometry by laser ablation. For a thin-film metal resistor, resistance is determined by the resistor's composition and physical dimensions. Laser trimming alters the shape of the resistor, which in turn alters the resistance. For example, a lateral cut in the resistor material by the laser narrows the current flow path and increases the resistance value. One advantage of laser trimming is the permanence of the process. In most cases, automated laser trimming only requires a one-time adjustment, so the process is less susceptible to error and re-work. Other advantages include high precision and reliability. Laser trimming, however, has some disadvantages as well. The cost of buying and operating laser trimming systems can be extremely high, and the process itself can be time-consuming. Laser trimming is also not useful for polysilicon resistors.
Thus, there exists a need for a polysilicon resistor that can be adjusted to a precise value in a cost-effective manner.